Aerogel insulation panels and manufacturing thereof

ABSTRACT

The present invention provides articles and methods related to insulation panels made from aerogels, and specifically polyimide based aerogels. Such insulation panels have a wide variety of applications, including specifically in aerospace applications.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser.No. 61/774,660, filed on Mar. 8, 2013; which is incorporated herein inentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractW31P4Q-10-C-0209 awarded by DARPA. The Government has certain rights inthis invention.

FIELD OF INVENTION

The invention relates to laminate panels which can be used in aerospaceapplications. The panels include a polyimide-based aerogel layer, atleast one facesheet layer, and a reflective protection layer on thefacesheet.

BACKGROUND

This invention is directed to laminate panels which include apolyimide-based aerogel layer, at least one facesheet layer, and areflective protection layer on the facesheet. The aerogel layerfunctions both to insulate and structurally support the panel.

Aerospace panels are difficult to design and produce. The nature ofaerospace travel presents environmental stress on panel materials whichare far beyond the stress faced in typical applications of thosematerials. Materials used in the design of aerospace paneling musttherefore be durable enough to withstand the extreme conditions in thosestressful environments. The nature of aerospace travel also requiresthat paneling materials be as light as possible. Small differences inthe density and weight of these materials can have significant effectson the functionality of the panel, as well as the functionality andenergy efficiency of the aerospace device as a whole. Thus, structuraland insulating materials in aerospace panels must be selected andoptimized for the specific challenges found in aerospace travel.

Aerogel composites have physical and chemical properties which canpotentially be optimized to withstand the demands of aerospace paneldesign. Aerogels describe a class of materials based upon theirstructure; namely low density, open cell structures with large surfaceareas (often 900 m2/g or higher) and sub-nanometer scale pore sizes.Aerogels can be prepared by replacing the liquid solvent in a wet gelwith air, without substantially altering or collapsing the networkstructure (e.g., pore characteristics) or the volume of the gel body.Supercritical and subcritical fluid extraction technologies are used toextract the fluid from the gel without causing the collapse of thepores. A variety of different inorganic and organic aerogel compositionsare known. Inorganic aerogels are generally based upon metal alkoxidesand include materials such as silica, carbides, and alumina. Organicaerogels include carbon aerogels and polymeric aerogels such aspolyimides.

Aerogels function as thermal insulators primarily by minimizingconduction (low density, tortuous path for heat transfer through thenanostructures), convection (very small pore sizes minimize convection),and radiation (IR suppressing dopants may easily be dispersed throughoutthe aerogel matrix). Depending on the formulation, aerogels can functionat temperatures of 550° C. and above. Low to moderate density aerogelmaterials (typically in the range of about 0.01 g/cm to about 0.3 g/cm)are widely considered to be the best solid thermal insulators, and havethermal conductivities of about 12 mW/m-K and below at 37.8° C. andatmospheric pressure.

Aerogels can also be fiber reinforced during production to providesignificant structural stability and resilience, particularly in highflexural strain applications. The flexibility of thin aerogel sheets(typically between about 0.1 mm and about 25 mm) allows for themanufacture of large sections of aerogel composites which retain most ofthe useful qualities of aerogels, such as low density and low thermalconductivity.

A need thus exists for structural and insulating materials which havethe strength, thermal conductivity, and density properties to allow foroptimized design and manufacturing of aerospace panels. Specifically, aneed exists for optimized aerogel materials which have strength, thermalconductivity, and density properties that allow for effective design andmanufacturing of aerospace panels. A need also exists for laminateaerospace panels which effectively incorporate the optimized aerogelmaterials.

SUMMARY OF THE INVENTION

The present invention describes a laminate panel comprising: (a) apolyimide based aerogel layer having two major faces; (b) a facesheet onat least one major face of the aerogel layer; and (c) a reflective layeradjacent to the facesheet. The panel may include a facesheet on bothmajor faces of the aerogel layer. The panel may additionally includeedge seals to seal the edges of the panel. The panel may comprise atleast one hole in or through the panel to facilitate fastening.

The panel includes a polyimide based aerogel layer. The polyimide basedaerogels used in the present invention may be carbonized. Carbonizationmay be carried out by pyrolysis at elevated temperatures in an inertatmosphere. The carbonized forms of the aerogels used in the presentinvention may have the nitrogen content between 0 and 20%. The aerogelsused in the present invention may also comprise silica components. Suchsilica components may be in physical mixtures with polyimides orcovalently linked to silica. The aerogel layer may also be reinforcedwith fibers. The aerogel may comprise an opacifier such titaniumdioxide, silicon carbide, carbon black, graphite phosphates, borates,metal silicates, metallocenes, molybdates, stannates, hydroxides,carbonates, zinc oxides, aluminum oxides, antimony oxides,magnesium-zinc blends, magnesium-zinc-antimony blends, or a combinationthereof.

The aerogel layer may comprise a polyimide based aerogel which hasspecific physical and chemical properties. Specifically, the arogellayer of the present invention can have a specific combination offlexural strength, flexural modulus, density, and thermal conductivity.The aerogel material can have a flexural strength, without failure, ofat least 150 psi, at least 200 psi, or at least 250 psi. The aerogelmaterial can have a flexural modulus, without failure, of at least 5,000psi, at least 20,000 psi, or at least 40,000 psi. The aerogel materialcan have a density below 0.25 g/cc, below 0.15 g/cc, or below 0.10 g/cc.The aerogel material can have a thermal conductivity at ambienttemperature and ambient pressure below 40 mW/m-K, below 30 mW/m-K, orbelow 20 mW/m-K. The aerogel material can have a thermal conductivity atambient temperature in a vacuum of 10⁻⁵ torr below 10 mW/m-K, below 5mW/m-K, or below 3 mW/m-K.

The panel includes a facesheet on at least one major face of the aerogellayer. In one embodiment, the panel includes a facesheet on both majorfaces of the aerogel layer. In another embodiment, the face sheetcomprises a graphite cyanate ester face sheet.

The panel includes a reflective layer adjacent to the facesheet. Thereflective layer reflects at least one frequency of incident energy. Inone embodiment, the reflective layer covers the entirety of thefacesheet. In another embodiment, the reflective layer protects thepanel from damage by reflecting at least one frequency of incidentenergy which can potentially damage the panel. The reflective layer cancomprise a silver Teflon coating material.

The panel may include at least one vent that allows air to flow in andout of the panel. The panel may include at least one filter that filtersparticulate matter. In one embodiment, the panel includes a vent thatallows air to flow in and out of the panel, and a particulate filterover the vent that keeps particulate matter within the panel. In anotherembodiment, the panel includes edge seals which include at least onevent and at least one particulate filter. In a further embodiment, thepanel includes edge seals which include at least one vent and at leastone particulate filter, wherein the vents in the edge seals allow air toflow in and out of the panel edges, and wherein the filters capture andretain particulate matter in the flowing air to prevent contamination ofthe air outside the panel with particulate matter.

The present invention also provides methods of manufacturing a laminatepanel comprising the steps of: (a) providing a polyimide aerogel layerwith two major faces; (b) attaching a facesheet to at least one majorface of the aerogel layer; and (c) attaching a reflective layer to thefacesheet. The panels of the present invention may be sealed at theedges. At least a hole through the panel may be provided for variousreasons, including accommodation for structural members. Such holes maynot have to be through the entire thickness of the panel and may simplyallow for fasteners or screws to be used for attachment of the panelagainst other components.

In one embodiment, the present invention comprises an aerogel insulationlayer having: (a) density of 0.20 g/cc or less, 0.15 g/cc or less, or0.10 g/cc or less; (b) a thermal conductivity at ambient temperature andambient pressure of 30 mW/m-K or less, 25 mW/m-K or less, 20 mW/m-K orless, or 15 mW/m-K or less; (c) a flexural strength of 150 psi or more,200 psi or more, or 250 psi or more; and (d) a flexural modulus of 5000psi or more, 20000 psi or more, or 40000 psi or more. The aerogelinsulation layer can also have a thermal conductivity at ambienttemperature and vacuum pressure at 10⁻⁵ Torr of 10 mW/m-K or less, 5mW/m-K or less, or 3 mW/m-K or less.

In another embodiment, the aerogel insulation layer comprises apolyimide aerogel or derivative thereof. The aerogel insulation layercan also be a fiber reinforced aerogel.

In yet another embodiment, the present invention comprises a laminatepanel comprising: (a) an aerogel layer having at least two major faces;(b) a facesheet adjacent to at least one major face of the aerogellayer; and (c) a reflective layer adjacent to the facesheet. The panelcan include a first facesheet adjacent to a first major face of theaerogel layer, and a second facesheet adjacent to a second major face ofthe aerogel layer. The panel can also be sealed at the edges by at leastone edge seal.

In still another embodiment, the panel includes at least one vent, andat least one vent filter. The panel can also include an edge seal whichincludes at least one vent and at least one vent filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the synthetic scheme for making polyimides.

FIG. 2 illustrates a variety of polyimide aerogel form factors:monolith, molded monolith, machine monolith, opacified monolith, andfiber reinforced.

FIG. 3 illustrates compressive strength of wet polyimide gel andgelation time versus catalyst concentration.

FIG. 4 describes non-carbonized green (g) and annealed (a)carbon-opacified PI aerogel monoliths.

FIG. 5 illustrates the thermal conductivity versus final density foropacified and unopacified PI aerogels.

FIG. 6 illustrates polyimide aerogel monolith before and after uniaxialcompression.

FIG. 7 illustrates compressive strength versus strain data for polyimideaerogel monoliths.

FIG. 8 illustrates compressive modulus versus density for polyimide,silica and silica-hybrid aerogel monoliths.

FIG. 9 illustrates pictures of PI reinforced aerogels.

FIG. 10 illustrates density and thermal conductivity data for fiberreinforced polyimide aerogel monoliths.

FIG. 11 illustrates thermal conductivity, thickness and density for 1,2, 3 and 4-ply stack-ups of fiber reinforced polyimide aerogels.

FIG. 12 illustrates a plot of thickness and density of multi-plystack-ups of fiber reinforced polyimide aerogels.

FIG. 13 illustrates photograph of PI aerogel carbon fiber compositesprepared at various target densities.

FIG. 14 illustrates final thickness, density and thermal conductivityfor carbon fiber reinforced polyimide aerogels prepared at varioustarget densities.

FIG. 15 illustrates SEM images of PI aerogel carbon fiber composite ata) 30×, b) 250× and c) 150 k× magnifications.

FIG. 16-1 includes a photograph of 4-ply PI aerogel carbon fibercomposite quilted using Kevlar thread.

FIG. 16-2 illustrates compressive and flexural strengths for multi-plycarbon fiber reinforced polyimide aerogels.

FIG. 17 illustrates compressive stress versus strain curves for variousfiber reinforced polyimide aerogels.

FIG. 18 illustrates flexural stress versus strain curves for variousfiber reinforced polyimide aerogels.

FIG. 19 illustrates vacuum-level thermal conductivity versus temperatureof monolith and carbon fiber reinforced polyimide aerogels.

FIG. 20 illustrates a schematic and prototype of sandwich compositepanel equipped with inserts and edge-seals containing carbon fiberreinforced polyimide aerogel.

FIGS. 21a-21k illustrate steps for installing blind insert into sandwichcomposite assembly.

FIG. 22 illustrates a schematic for the segmented edge-seal design.

FIG. 23 illustrates 3″×8″ panel coupons after 4-point bend testing,indicating delamination at the edges between facesheets and PI aerogelcore.

FIG. 24 illustrates inter-laminar failure for 2″×2″ samples duringflatwise tensile testing.

FIG. 25 illustrates inter-laminar failure for 3″×4″ samples during sheartesting.

FIG. 26 illustrates stress-strain curves for 2″×2″ samples duringflatwise compression.

DETAILED DESCRIPTION

The present invention provides a laminate panel comprising: (a) apolyimide based aerogel layer having two major faces; (b) a facesheet onat least one major face of the aerogel layer; and (c) a reflective layeradjacent to the facesheet. The panel may include a facesheet on bothmajor faces of the aerogel layer. The panel may additionally includeedge seals to seal the edges of the panel. The panel may comprise atleast one hole in or through the panel.

The polyimide aerogels can be optimized to have specific physical andchemical properties. Specifically, the aerogels of the present inventioncan be optimized to have a specific combination of flexural strength,flexural modulus, density, and thermal conductivity. The aerogelmaterial can have a flexural strength, without failure, of at least 150psi, at least 200 psi, or at least 250 psi. The aerogel material canhave a flexural modulus, without failure, of at least 5,000 psi, atleast 20,000 psi, or at least 40,000 psi. The aerogel material can havea density below 0.25 g/cc, below 0.15 g/cc, or below 0.10 g/cc. Theaerogel material can have a thermal conductivity at ambient temperatureand ambient pressure below 40 mW/m-K, below 30 mW/m-K, below 20 mW/m-K,or 15 mW/m-K or less. The aerogel material can have a thermalconductivity at ambient temperature in a vacuum of 10⁻⁵ torr below 10mW/m-K, below 5 mW/m-K, or below 3 mW/m-K.

The flexural strength of a material is the ability of a material toresist catastrophic deformation under a loaded stress. Flexural strengthcan be measured using a three point transverse-bending flexural testtechnique, such as the ASTM D790 test. Generally, the flexural strengthis defined by the pressure (psi or Mbar) experienced by a material atthe moment of rupture or catastrophic physical event due to loadedstress. This value is typically extracted from a stress-strain curveproduced by a flexural strength test for a material (See FIG. 18).

The flexural modulus of a material is the tendency for a material tobend, or more specifically the ratio of stress to strain in flexuraldeformation of the material. It is determined from the slope of astress-strain curve produced by a flexural strength test (such as theASTM D790).

The laminate panel can also include an edge seal. The edge seal can becontinuous along the entire edge of the panel. The edge seal can consistof a segmented design (FIG. 22). In one embodiment, the segmented designcan include 2″-long, U-shaped fiberglass seal segments which aredistributed along the edge of the panel. The seal segments include 0.25″vents covered by 0.5 μm particle filters. The vents allow air to pass inand out of the panel. This venting is important in aerospaceapplications because it allows the air pressure inside the panel tonormalize with the ambient pressure of the environment as the paneltraverses between various elevations. The particle filter prevents anyparticles from escaping the panel interior during launch evacuationand/or panel outgassing. These segments are bonded to the outsidefacesheets with a 2″ gap between the ends of the segments. The 2″ gapbetween segments is covered with 2-mil Kapton tape to complete the sealof the panel edges. This design offers some significant fabricationadvantages in that the 2″ long segments of formed edge seal can bepre-made, cut to 2″ lengths and inventoried.

The present invention also provides processes for the production offiber reinforced polyimide aerogel composites. The process includes: (a)forming poly(amic acid) solution from a mixture of dianhydride anddiamine monomers in a dipolar, basic solvent such asN,N-dimethylacetamide (DMAc) or N-methylpyrolidone (NMP); (b) castingthe poly(amic acid) solution into a fiber batting prior to gelation; (c)gelling the poly(amic acid) solution by chemical imidization usingacetic anhydride and pyridine; and (d) removal of the transient liquidphase contained within the gel with supercritical or subcritical CO2 togive a fiber/polyimide aerogel composite. The polyimide aerogels can beproduced from aromatic dianhydride and diamine monomers, such asaromatic diamines or a mixture of at least one aromatic diamine monomerand at least one aliphatic diamine monomer. The resulting polyimideaerogel can be optimized to possess low density, meso-pores, narrow poresize distribution and good mechanical strength.

The preparation of polyimide wet gels is a two-step procedure (FIG. 1):(a) formation of the poly(amic acid) solution from a mixture ofdianhydride and diamine in a dipolar, basic solvent such asN,N-dimethylacetamide (DMAc) or N-methylpyrolidone (NMP); and (b)catalyzed cyclization with acetic anhydride and pyridine to formpolyimide. Previous work with synthesizing polyimide aerogels has shownthat the first step typically requires at least 12 hours mixing at roomtemperature allowing for significant formation of the poly(amic acid)polymer and yielding stable, robust wet gels. Gelation conditions dependon several factors, including the prepared density of the solution.Higher density solutions will gel faster than lower density solutions.Once the system has reached the gelled state, the gels are heat treatedin an oven at 65° C. for 16 hours after which time the wet gels arerinsed repeatedly with ethanol. Rinsing occurs at least three timesprior to CO₂ extraction, and serves to remove residual DMAc solvent andunreacted monomers which are insoluble in neither super- nor subcriticalCO₂. CO₂ can then be used in techniques known to those in the art forwet solvent extraction to create the aerogel structure. Furthertechniques for preparing and optimizing polyimide aerogels can be foundin U.S. patent application Ser. Nos. 10/625,501 and 11/047,200, both ofwhich are incorporated herein by reference in their entirety.

The polyimide aerogels can take a variety of forms (FIG. 2). When thepoly(amic acid) solution is catalyzed using acetic anhydride andpyridine, the resulting wet gel and aerogel is transparent andorange-red in color. When catalyzed using triethylamine, the resultingwet gel and aerogel are opaque and yellow in color. Polyimide aerogelscan be prepared as free-standing transparent monoliths which can bemolded to specific shape or machined in the final aerogel state.Polyimide aerogels can also be reinforced with a continuous fiberbatting to provide a stiff board composite material. Finally, thepoly(amic acid) solution prior to gelation can be doped with a varietyof opacifiers, including carbon black, graphite, carbon nanotubes, metaloxides, metals, titanium dioxide, silicon carbide, phosphates, borates,metal silicates, metallocenes, molybdates, stannates, hydroxides,carbonates, zinc oxides, aluminum oxides, antimony oxides,magnesium-zinc blends, magnesium-zinc-antimony blends, or a combinationthereof.

In one embodiment, silica components are also present in the aerogels ofthe present invention. Silica may be incorporated into the polyimidematrix either as physical mixture or it may be bound covalently to thepolyimide. U.S. patent application Ser. No. 13/299,677 describes some ofsuch covalently bonded silica-polyimide hybrid aerogels, which isincorporated by reference in its entirety. In another embodiment, thepolyimide aerogels of the present invention are carbonized.Carbonization results in at least some of the polymeric materials beingconverted to carbon using various known techniques, such as pyrolysis.In a preferred embodiment, the polyimide aerogels are heated to elevatedtemperatures sufficiently high to induce pyrolysis. Typical pyrolysistemperatures range between 500° C. and 2000° C. Temperature may beincreased to reduce the nitrogen content of the resulting carbonaerogel. Pyrolysis is typically carried out in an inert atmosphere (i.e.nitrogen, helium, neon, argon or some combination). The nitrogen contentof the carbon aerogels may be from 0% to 20%.

The present invention further provides a processes for fabricatinglaminate, sandwich panel composites comprised of fiber reinforcedpolyimide aerogels composites and facesheets. The panels can alsoinclude additional elements such as reflective protection layers,edge-seals, vents, filters, and inserts for component mounting. Theprocess includes: (a) providing an aerogel layer having two faces, andat least one facesheet; (b) cleaning and optionally abraiding thesurfaces of the aerogel and facesheet to prepare for adhesion; (c)applying a film adhesive to the facesheet; (d) combining the facesheetand aerogel layer so the facesheet adheres to the face of the aerogellayer; and (e) curing the aerogel-facesheet laminate to completeadhesion.

The following examples provide various embodiments and properties of thepresent invention.

Example 1

The gel strength of polyimide wet gel was measured for a system preparedwith initial target density (i.e. solids content) of 0.085 g/cc as afunction of the catalyst concentration. Catalyst content in FIG. 3represents the combined molar ratio of acetic anhydride and pyridine.The wet gel compressive strength measured after 24 hours of syneresiswas found to be approximately 4600 psi for catalyst contents rangingbetween 9.2 and 4.4. When the catalyst concentration was 2.8, the geltime was considerable longer and the resulting wet gel strength wasnearly half (1900 psi) as those with higher catalyst contents.

Example 2

Carbon black (from Cabot-Monarch 1300) was used as an opacifier in thepreparation of polyimide aerogel monoliths in order to reduce thethermal conductivity. Opacifiers with sub-micron particle size arepreferably used as an additional heat dissipating material. Thepoly(amic acid) sol as described previously that was used to prepareunopacified PI aerogel monoliths was also employed for the synthesis ofcarbon-opacified PI aerogel monoliths. Target densities of 0.03, 0.06and 0.08 g/cc were prepared with 1 and 5 wt % carbon. It is notpreferable to add the opacifier at earlier stages of PI formation as itmay result in obstructed polymerization growth, and weak gels.Preferably, adding the opacifier at 1 and 5 wt % (relative to totalsolid content) after acetic anhydride but before pyridine catalysts,helps preserving the integrity of the polymerization. To preventpossible sedimentation of the carbon prior to gelation, the solutionswere ultrasonicated for 15 minutes, and after the gels were casted, theywere immediately heated at 65° C. for 16 hours (overnight). Likewise,the wet gels were rinsed three times with ethanol prior to CO₂extraction. Pictures of carbon-opacified PI aerogels can be seen in FIG.4.

Opacification led to nearly a 10 unit decrease in thermal conductivitywhen 5 wt % carbon was employed. This is evident in FIG. 5 and Table 1.

TABLE 1 Physical Characteristics of Carbon- Opacified PI AerogelMonoliths. Target Final Thermal Density Density Conductivity (g/cc)(g/cc) (mW/m-K) 1%-3g 0.03 0.0633 18.1 1%-6g 0.06 0.1377 30.1 1%-8g 0.080.1909 43.0 5%-3g 0.03 0.0549 21.0 5%-6g 0.06 0.1614 23.4 5%-8g 0.080.1845 34.0 1%-6a 0.06 0.1633 37.1 1%-8a 0.08 0.2425 56.5 5%-8a 0.080.2174 45.3

For instance, 1%—8 g had a TC of 43.0 mW/m·K. 5%—8 g had nearly the samefinal density but a TC value of 34.0 mW/m·K. For those samples that wereannealed, 1%—6a, 1%—8a and 5%—8a, the result is consistent withunopacified PI aerogel monoliths in that density increases afterannealing leading to an increase in TC. This demonstrates the potentialfor opacification to reduce thermal conductivity at least at ambientconditions. It is expected that this observation would be more dramaticat elevated temperatures where contributions to radiant heat transferare greater. Opacification above 5 wt % carbon tended to decrease thestrength of the aerogels, especially at lower densities, and wastherefore not considered for further study.

Example 3

Uniaxial compression analysis of the samples of the present inventionwas tested using an Instron (Model 5569) with a 50 kN load cell andcrosshead speed of 0.050″/min. Stress-strain curves were determined foreach density of PI aerogel monolith and compressive Young's modulus, E,was calculated. When PI aerogel monoliths are compressed uniaxially,their compression profiles are comparable to organic foams. At lowerstrains, the initial slope (between ˜2 and 5% strain) is used tocalculate Young's modulus. It is believed that at this stage ofcompression, the monolith possesses elastic properties. Past the initialslope, the sample exhibits a plateau where, as strain increases, thestress minimally increases. At this compressive stress the cellsirreversibly collapse, and as long as there is a narrow distribution incell size, the plateau region shall remain relatively horizontal. As thecells completely collapse with continued strain, opposing cell wallscompress and solid pushes against solid. This is reflected in a rapidincrease in stress and is commonly referred to as densification. FIG. 6shows a PI monolith before and after compression.

Example 4

The compressive strengths for PI-4, PI-5, PI-6, PI-8, PI-4a, PI-6a andPI-8a were measured from their corresponding stress-strain curves shownin FIG. 7 and listed in the accompanying table. The entire stress-straincurve for each of the densities reveals the aerogel's unique ability notto splinter, crack, or break during a compressive load. Compared withsilica aerogels which are extremely dusty and very brittle, this is animportant characteristic when considering custom applications thatrequire machining and minimal particle shedding. The Young's modulus wascalculated from the slope over the range of 2% and 5% strain. It isevident from the data that as density increases so does the Young'smodulus and the onset of cell collapse. It appears to be consistent thatafter about 5% strain, regardless of density, pore collapse begins.

The typical relationship between compressive modulus and density followsa power law function. As seen in FIG. 8 the strength of PI aerogelsincreases at a faster rate, with respect to a rising density, than anyother silica or silica hybrid systems.

Example 5

Reinforcing PI aerogels was considered as a means to increase theoverall mechanical strength and ease of handling while still maintainingfinal densities within program requirements. As will be described later,reinforcement into polyimide aerogel not only increased the flexuralstrength but also lowered the thermal conductivity (TC) from theirparent unopacified, non-reinforced PI aerogel monoliths. Three battingmaterials—or substrates—were chosen for this study: carbon felt (CF)with a density of 0.094 g/cc and measured TC of 34.4 mW/m·K (hot-platetested under ambient conditions at room temperature), Quartzel® (fromSaint-Gobain, abbreviated here as Q) with a density of 0.0069 g/cc andmeasured TC of 38.6 mW/m·K, and Carbon Tissue (from Technical FibreProducts, Inc.; abbreviated as CT) with a density of 0.0396 g/cc andmeasured TC of 27.2 mW/m·K.

Reinforced PI aerogels were prepared by making polyimide solutions withinitial target densities 0.03, 0.04, 0.06 and 0.08 g/cc. Depending onthe initial density and size of substrate, a predetermined volume ofpolyimide solution was prepared in order to completely fill thesubstrate. After acetic anhydride and pyridine were sequentially addedto the poly(amic acid) solution, depending on the predetermined geltime, the solutions were casted directly into the substrates containedwithin a polypropylene mold. The PI solution easily penetrated throughthe fibers, which did not interfere with the expected times of gelation.Once the solutions gelled, the wet gels were heated at 65° C. for 16hours to allow for gel strengthening. Prior to CO₂ extraction, the gelswere rinsed at least three times daily with ethanol in order to removeresidual solvent, catalysts, and unreacted monomers. Pictures of greenand annealed reinforced PI aerogels can be seen in FIG. 9.

When PI aerogels are reinforced with Carbon Felt, Quartzel®, or CarbonTissue, we see a general decrease in thermal conductivity (FIG. 10).Although with Carbon Felt we saw a considerable decrease in thermalconductivity (TC ranging from 18.5 to 23.6 mW/m·K), the final densitywas too far above the desired target density for the program, CF-4g withfinal density of 0.1159 g/cc. Additionally, this sample exhibitedsignificant shrinkage and cracking post extraction and annealing due toa non-uniform shrinking of the substrate with respect to the PI aerogel.Quartzel®, a much lighter fiber, however, exhibited low final densitiesand low thermal conductivities (19.0 to 24.6 mW/m·K) but also shranknon-uniformly. The quality of this final composite was generally weakbut flexible, characteristics not required for a structural satellitepanel. An aerogel which utilizes this substrate may be a strongcandidate for other applications requiring flexibility and good thermalperformance. Lastly, Carbon Tissue demonstrated low densities, lowthermal conductivities and no shrinkage during extraction and annealing.One example is CT-4g with a final density of 0.0936 g/cc and thermalconductivity of 18.6 mW/m·K. Carbon tissue is a batting formed from longcarbon-based fibers that primarily run in a planar manner in the lateralx- and y-directions. This gives Carbon Tissue its remarkable rigidity.As a result, Carbon Tissue was chosen as the downselected substrate tobe used for further reinforcement experiments.

Example 6

To increase thickness of the aerogel/fiber composites we pre-laid up6″×6″ carbon fiber battings in 1-, 2-, 3- and 4-ply thicknesses. Thepoly(amic acid) solution with a target density of 0.045 g/cc was castedatop the multi-ply layups and allowed to gel. The samples are then aged,rinsed and supercritically dried to afford the samples shown in FIG. 11.Prior to any thermal or mechanical testing, the composites were heattreated at 200° C. for 20 minutes to complete imidization and drive offany residual solvents. After each of these steps there was no evidenceof inter-ply or ply-ply delamination, suggesting a continuous PI aerogelnetwork through the composite.

For each prepared multiple-ply layup (described above), the density andthermal conductivity were measured for each composite at ambienttemperature and pressure. The final densities for all plies were allwithin error of each other, 0.12 g/cc. Weight and thickness increaselinearly with the number of plies. A single ply composite exhibitedfinal thickness of approximately 5 mm. To achieve a ¾″ ITS panel corewould require a 3 or 4-ply layup, 0.63″ and 0.81″, respectively. Thermalconductivity was also not effected by increasing the number of plies. Asingle ply layup had a thermal conductivity of 21.8 mW/m-K, but as thenumber of plies increased to 2, 3 and 4 plies, the TC did not changesignificantly, 20.7, 19.3, 20.1 mW/m-K, respectively. These trends anddata are demonstrated in FIG. 12.

Example 7

A series of PI aerogel carbon fiber 6″×6″ composites were preparedaccording to the same methods as described previously but with highertarget densities, i.e. with greater solid contents in the poly(amicacid) sol prep (FIG. 13). We were motivated to increase the density asan effort to increase the overall strength of the composite. Accordingto target density studies, the compressive strength of monolithic PIaerogel samples increases exponentially. While using the same carbonfiber reinforcement, target densities of 0.04, 0.05, 0.06 and 0.07 g/ccwere prepared.

FIG. 14 summarizes the physical characteristics of these composites. Thesample with a target density of 0.04 g/cc was prepared as a control witha thickness of 5.06 mm and final density of 0.120 g/cc. By increasingthe target density, we see a slight decrease in the final thickness andan expected increase in final density. About a 0.5 mW/m-K increase ofthermal conductivity was observed for each unit increase in the targetdensity.

Example 8

The PI aerogel carbon fiber composite was investigated by scanningelectron microscopy (SEM) in order to understand the continuity of theaerogel network within the composite and the interface between aerogeland fiber. It is believed that composite materials where the connectionbetween multiple components is strong will exhibit greater mechanicalstrength. Discontinuities in the form of gaps or micro-cracking resultin diminished mechanical integrity. According to the SEM images seen inFIG. 15 of a PI aerogel/carbon fiber composite, at 30× magnification itis evident that the PI aerogel material is continuous throughout thefiber. However, at closer inspection at 250× magnification, there is anobvious lack of connectivity between the carbon fiber reinforcement andthe PI aerogel. This behavior is reflected in the poor transversecompressive and tensile strengths observed for this material. Becausethe carbon fibers in the batting are predominantly in the x,y plane andvery stiff and therefore high flexural strength, when the PI gelundergoes shrinkage, the aerogel shrinks away from the fiber leavingbehind gaps at the micron level. It is obvious that during a flatwisecompression test, a sample will exhibit low compression strength,especially at low composite density. The image at 150 k× illustrates thefibrillar network typical for PI aerogels at Aspen Aerogels.

In order to increase the tensile strength in the z-direction, we quilted(by hand) a multiple-ply layup using Kevlar thread (size 46) in across-stitch pattern as seen in FIG. 16. The quilted composite samplewas noticeably stronger. When tested for flexural strength, a 13%increase strength was observed. FIG. 16 proves that ply-ply shearstresses were alleviated by quilting and therefore sustained higherflexural loads.

Example 9

Uniaxial compression analyses of the reinforced samples described abovewith TD=0.04 g/cc were examined using an Instron (Model 5569) with a 50kN load cell and crosshead speed of 0.050″/min. Stress-strain curveswere determined for each reinforced PI aerogel and compressive Young'smodulus, E, was calculated.

The compressive strengths for CF-4g, CF-4a, Q-4g, Q-4a, CT-4g, CT-4awere measured from their corresponding stress-strain curves shown inFIG. 17 Compared with silica aerogels, which are extremely dusty andvery brittle, non-reinforced and reinforced PI samples show minimalparticle shedding and dust. A Young's modulus and maximum compressivestress were determined for each sample and summarized in FIG. 17.Overall, reinforcement of PI aerogels leads to lower thermalconductivities at the expense of lower maximum compressive stresses andlower Young's moduli. A non-reinforced PI aerogel monolith prepared withTD=0.04 g/cc has a maximum compressive strength of 57.3 psi and acompression modulus of 816.7 psi. When a PI aerogel of the same densityis reinforced with carbon felt, Quartzel®, and Carbon Tissue, themaximum compressive strengths are 11.7, 10.7 and 11.5 psi andcompression moduli of 91.8, 90.4 and 182.4 psi. The Carbon Tissuesubstrate afforded the highest compressive strengths amongst all of thefibers due in part to its unique fiber layout. As expected, the annealedsamples exhibited higher strengths, a reflection of the increaseddensity upon heating.

Example 10

Mechanical analysis of the same reinforced PI aerogels also includes3-point flexural testing according to the design outlined by ASTM D790.Flexural testing provides insight to alternative stresses other thancompression. For example, when a panel is flexed, the sample undergoesboth tension and compression stresses but in the lateral (x,y)direction. These stresses cannot be measured in a uniaxial (z-direction)compression test alone. Stress-strain curves were measured for the greenand annealed reinforced samples with TD=0.04 g/cc (FIG. 18), and thedata is summarized in FIG. 18. In contrast to the compressive strengthof the reinforced PI aerogels, the flexural strength in most instancesexceeded that of the non-reinforced PI aerogel monolith. The CarbonTissue substrate exhibited the greatest flexural strength of 186 psi anda Young's modulus of 26,892 psi. The combined flexural strength and lowthermal conductivity makes CT-4g a prime candidate for furtherdevelopment.

The fiber reinforced PI aerogel composite exhibited excellent thermalconductivity in a vacuum of 10⁻⁵ torr. FIG. 19 shows how this materialcompares with non-reinforced polyimide aerogel in vacuum over thetemperature range of 193 K up to 423 K (−80° C. to 150° C.). As can beseen, the thermal conductivity of this material was measured at a verylow 1.5 mW/m-K at a sample temperature of 193 K (−80° C.) and then roseto 4.5 mW/m-K at 423 K (150° C.). In looking at FIG. 19, we believe thatthe dramatic drop in thermal conductivity in the carbon vale (a thinnervariant of carbon tissue) material was due to the cumulative opacifyingeffect of the carbon vale reinforcement with the addition of the 5%carbon black opacifier further improving the results. By adding theseopacifiers to the polyimide aerogels, the radiative component of heattransfer was greatly reduced; thereby, bringing the overall thermalconductivity down to a range of 1.5 mW/m·K at 193 K up to 4.5 mW/m·K at423 K, as shown in FIG. 19.

Example 11

A pictorial representation of a sandwich panel composite, where thestructural and thermal functions are integrated into a single component,is shown in FIG. 20, along with pictures of 14″×14″ manufacturedprototype panel. This sandwich panel was manufactured by a ply to plybonding process of the fiber reinforced polyimide aerogel core materialwith low thermal conductivity (3.3 mW/m-K at 10⁻⁵ ton at 20° C.) andgood mechanical properties (flex strength ˜700 psi and shear strength of120 psi) to facesheets with adhesive, an array of blind and thru-holeinserts were incorporated, along with a segmented edge-seal close-outsystem.

The following description provides details of a specific embodiment ofthe manufacturing of laminar panels of the present invention.

Materials:

Substrates to be bonded are PI aerogel carbon fiber composite andgraphite-cyanate ester (Gr/Ce) skins for sandwiched samples. The aerogelcan be single layer or multiple layers. A film adhesive with density of0.030 lbs./ft² FM300-2U, which is space-qualified is used in thisembodiment.

Surface Preparation:

All bonding surfaces were initially cleaned. Gr/Ce surface was abradedto a dull mat finish and wiped clean with alcohol moistened alcoholwipes until wipe comes clean and kept clean during the bonding process.Blot the bonding surfaces with alcohol moistened wipe as required toremove any incidental surface contaminants. Gently remove any looseaerogel debris from cut edges as needed. Take care to not knock looseadditional debris.

Sandwich Bond/Cure Process:

-   -   1. Wipe Gr/Ce skins with alcohol moistened wipe until wipe comes        clean. Allow 10 minutes for alcohol to flash at RT.    -   2. Remove any loose debris of aerogel on core edges and if        required, blot core with alcohol moistened (not saturated) wipes        and allow for 20 minutes for alcohol to flash at RT. This is a        guide line; this material is very surface porous and may require        prolonged elevated temperature (120° F. for 30 minutes) to get        all alcohol to flash from pores. For core of this type, it is        best to wear proper protective gloves and clean surfaces that        come in contact with the core material to prevent any        contamination that will require cleaning prior to bond.    -   3. Apply film adhesive to skins. Use heat of hands to tack for        smaller sized samples or for larger samples, vacuum bag and        compact at full vacuum (˜12 psi) for 15 minutes. Sometimes areas        will be bare due to film adhesive fracturing and flaking off        before or after the adhesive application process. Repairs to        these areas can be made using the same film adhesive.    -   4. Stack up the sandwich laminate. As required, provide edge        dams to protect the sides of core from side pressure from vacuum        bag.    -   5. Bag for cure with appropriate temperature controls. Applying        full vacuum (˜12 psi) to sandwich stack up check for leaks and        cure.    -   6. Cure by ramping up at 3°-5° F. per minute until the        temperature reaches 250°±10° F. and hold at this temperature        range for 90 minutes minimum.    -   7. Cool down at 3°-5° F. per minute to lower than 150° F.    -   8. Vent vacuum and remove from tool.

Laminating Multilayer Core Bond/Cure Process:

-   -   1. Remove any loose debris of aerogel on core edges and if        required, blot core with alcohol moistened (not saturated) wipes        and allow for 20 minutes for alcohol to flash at RT. This is a        guide line; this material is very surface porous and may require        prolonged elevated temperature (120° F. for 30 minutes) to get        all alcohol to flash from pores. For core of this type, it is        best to wear proper protective gloves and clean surfaces that        come in contact with the core material to prevent any        contamination that will require cleaning prior to bond.    -   2. Apply film adhesive to core layers. Tacking is difficult on        this type of surface, perform repairs in bare spots as needed        using the same film adhesive. Full vacuum (˜12 psi) for 30        minutes to assure the adhesion.    -   3. Stack up the core multiply. As required, provide edge dams to        protect the sides of core from side pressure from vacuum bag.    -   4. Bag for cure with appropriate temperature controls. Applying        full vacuum (˜12 psi) to multilayered core stack up check for        leaks and cure.    -   5. Cure by ramping up at 3°-5° F. per minute until the        temperature reaches 250°±10° F. and hold at this temperature        range for 90 minutes minimum.    -   6. Cool down at 3°-5° F. per minute to lower than 150° F.    -   7. Vent vacuum and remove from tool.

A step-by-step procedure for installing a partial penetration, or“blind”, insert into a multi-ply lay-up of fiber reinforced polyimideaerogels (FIG. 21). Since the inserts are typically made of metal, inorder to eliminate direct heat leakage through the z-direction of thepanel, a method was developed to securely install inserts with partialpenetration into the thickness of the panel. This type of insert isco-cured with a pre-drilled facesheet and then installed as a singlecomponent into an already bored out aerogel core.

The edge seal consists of a segmented design (FIG. 22). In this design,the U-shaped seal segments were produced in 2″ long segments with 0.25″vents covered by 0.5 μm particle filters to prevent any particles fromescaping the panel interior during launch evacuation and/or paneloutgassing. These segments would then be bonded to the outsidefacesheets with a 2″ gap between the ends of the segments. This 2″ gapwould then be covered with 2-mil Kapton tape to complete the seal of thepanel edges. The wall thicknesses for these formed segments were 0.010″to minimize the amount of conducted heat leakage along the edge seal.This design offers some significant fabrication advantages in that the2″ long segments of formed edge seal can be pre-made, cut to 2″ lengthsand inventoried. Fiber glass was the chosen material for the formedsegments of this edge seal design because of its very low added heatleakage and reduced weight.

Example 12

Table 2 lists all of the mechanical tests (shear, flexural, compressionand tensile), the number of samples, and panel configurations for the PIaerogel sandwich composites. Two sets of 4-point bend flexural testswere performed, three 3″×8″ coupons each with and without the 5 cmrequired grid pattern and according to ASTM C-393 and included graphitecyanate ester facesheets, with no edge seals or thru-hole inserts. Sheartesting, according to ASTM C-273, was conducted on five aerogel coreonly samples having the dimensions 3″×4″×0.75″. Flatwise tensiletesting, according to ASTM C-297, was performed on aerogel core only2″×2″ samples. It was also of interest to understand how the thru-holeinserts would add to the tensile strength, so three 6″×6″ samples withfacesheets were prepared and tested having only one thru-hole insertinstalled at the center of the panel. Lastly, 2″×2″ core only sampleswere tested for flatwise compression according to the ASTM D695.

TABLE 2 Prepared Mechanical Test Coupons and Configurations. No. ItemTest ASTM Samples Dimensions Configuration 1 4-Point C-393 3 3″ × 8″ ×0.8″ Core + Bend facesheet, no edge seals or inserts 2 4-Point C-393 33″ × 8″ × 0.8″ Core + Bend facesheet, 5 cm grid of blind inserts, noedge seals or thru- hole inserts 3 Shear C-273 5  3″ × 4″ × 0.75″ Coreonly 4 Flatwise C-297 5  2″ × 2″ × 0.75″ Core only Tensile 5 FlatwiseC-297 3 6″ × 6″ × 0.8″ Core + Tensile facesheet, one thru-hole insert 6Flatwise D-695 5  2″ × 2″ × 0.75″ Core only Compres- sion

The first set of samples tested included three coupons each for 4-PointBend (according to ASTM C-393)—3 with no inserts and 3 with the 5 cmgrid of blind inserts. According to the data observed in Table 3, thereis little variability between measurements, and the installation ofblind inserts does not appear to affect the flexural behavior of thecoupon-sized panels. Maximum flexural loads were observed between about325 and 380 lbf (flexural load) with displacements of approximately−0.23 and −0.89 inches. The amount of displacement required to initiatecoupon failure indicates that the sandwich composite is fairly flexible.The method of failure in all cases was shear delamination at thefacesheet/core interface (FIG. 23).

TABLE 3 Results for 4-Point Bend Flexural Test. Coupon Flexural No. ofthick- load at max. Displace- Cou- blind ness, flexural ment at Failurepon inserts in. load (lbf) max (in) Mode 1 0 0.807 366.3 −0.700Faceskin/core interface 2 0 0.813 326.1 −0.707 Faceskin/core interface 30 0.798 339.2 −0.829 Faceskin/core interface 4 3 0.803 359.9 −0.829Faceskin/core interface 5 3 0.803 377.4 −0.887 Faceskin/core interface 63 0.803 372.6 −0.510 Faceskin/core interface

Flatwise tensile tests were performed according to ASTM C-297 with 0.75inch cores and no inserts. The results from this test are summarized in4. There is considerable variability between the collected data, withmaximum load data ranging between 12 and 45 lbf, (3 and 11 psi). For thesame target density of 0.05 g/cc, pressures of at least 25 psi wererequired to achieve interlaminar failure (FIG. 24).

TABLE 4 Results for Flatwise Tensile Test. Flatwise No. of Max. TensileMax Cou- blind Load Strength Extension Failure pon inserts (lbf) (psi)(in) Mode 1 0 12.802 3.186 0.047 Interlaminar 2 0 45.287 11.265 0.071Interlaminar 3 0 20.260 5.052 0.021 Interlaminar 4 0 14.075 3.506 0.040Interlaminar 5 0 23.667 5.878 0.035 Interlaminar

A total of five samples were prepared for shear testing (according toASTM C-273). No inserts or facesheets were included with these samples.The results are summarized in Table 5. Three out of the five samplesexhibited some form of delamination prior to testing. For samples 2 and3, the maximum loads required to cause core shear within the multi-plystack-up of aerogel composite occurred at 568 and 464 lbf (or 47 and 38psi, respectively). Compared to previous shear strength measurements on0.05 g/cc target density composites with the old carbon-fiber batting,maximum load values were between 1621 and 1858 lbf (134 and 153 psi). InFIG. 25 it is evident from the post core shear failure image thatbuilt-in irregularities of the carbon-fiber batting can lead topreliminary mechanical failure.

TABLE 5 Results for Core Shear Strength Test. No. of Max. Shear Max Cou-blind Load Stress Extension Failure pon inserts (lbf) (psi) (in) Mode 10 NA NA NA Preliminary failure 2 0 567.85 47.12 0.087 Core shear 3 0464.02 38.49 0.053 Core shear 4 0 70.60 5.83 0.021 Core shear(pre-failure)

Five 2″×2″ samples were measured for compression (according to ASTMD-695). No inserts, edge-seals or facesheets were installed for this setof samples. Stress-strain curves for all five samples are shown in FIG.26. Uniaxial compression analysis of these composites indicates that atlower strains, the samples reach a plateau followed by a moresubstantial increase in compressive load. During the plateau, theaerogel morphology irreversibly collapses, and as long as there is anarrow distribution in cell size, the plateau region remains relativelyhorizontal. As the cells completely collapse with continued strain,opposing cell walls compress and solid pushes against solid. This isreflected in a rapid increase in stress and is commonly referred to asdensification. The compressive strength for these samples is estimatedto be around 5 psi and is determined at initial strain and prior to theplateau portion of the curve. The strength of these composites is againweaker than expected due to non-uniform carbon-fiber battingirregularities.

Example 13

CT-4g was prepared using carbon fiber, manufactured by Technical FibreProducts Inc., as the reinforcement for the PI aerogel. The fiber has anoriginal density of 0.0396 g/cc and is approximately ¼″ in thickness. Inorder to obtain final core thickness of approximately ¾″, the PIaerogel/fiber composite was laid up into multiple plies. CT-4g wasprepared by first preparing the poly(amic acid) solution. Pyromelliticdianhydride (PMDA, 1 mol) is dissolved in a dipolar, basic solvent suchN,N-dimethylacetamide (DMAc) or N-methylpyrolidone (NMP) (4% solids).Once dissolved, 1,4-phenylene diamine (PDA, 1 mol) is then added to thesolution and allowed to dissolve and polymerize for 12 hours at roomtemperature. After the poly(amic acid) sol has been prepared, it isready to be catalyzed and casted in the carbon fiber reinforcement. Tothe poly(amic acid) sol, acetic anhydride (3.15 mol) is added andallowed to stir for 1 hour at room temperature. Pyridine (1.26 mol) isadded to the solution. Before the gelation point (20 minutes), thesolution is cast into the carbon-fiber reinforcement. After the PI gelis formed, the wet-gel fiber composite is aged 16 hours at 60° C. andsubsequently rinsed with ethanol to remove all DMAC, unreacted monomers,and residual catalysts. The wet-gel fiber composite is then subjected tosupercritical CO₂ drying to afford the PI aerogel carbon-fibercomposite.

What is claimed is:
 1. A fiber-reinforced aerogel composite, having: a)a density between 0.01 g/cc and 0.30 g/cc; b) a thermal conductivity atambient temperature and ambient pressure of between 15 mW/m-K and 30mW/m-K; and c) a flexural strength between 105 psi and about 275 psi. 2.The aerogel composite of claim 1, wherein the thermal conductivity ofthe aerogel composite at ambient temperature and ambient pressure isbetween 15 mW/m-K and 25 mW/m-K.
 3. The aerogel composite of claim 1,wherein the thermal conductivity of the aerogel composite at ambienttemperature and ambient pressure is between 15 mW/m-K and 20 mW/m-K. 4.The aerogel composite of claim 1, wherein the aerogel composite has athermal conductivity at ambient temperature and vacuum pressure at 10⁻⁵Torr of between 1 mW/m-K and 5 mW/m-K.
 5. The aerogel composite of claim1, wherein the flexural strength of the aerogel composite is betweenabout 150 psi and about 275 psi.
 6. The aerogel composite of claim 1,wherein the flexural strength of the aerogel composite is between about200 psi and about 275 psi.
 7. The aerogel composite of claim 1, whereinthe aerogel composite has a flexural modulus between about 5,000 psi andabout 46,000 psi.
 8. The aerogel composite of claim 7, wherein theflexural modulus of the aerogel composite is between about 15,000 psiand about 46,000 psi.
 9. The aerogel composite of claim 7, wherein theflexural modulus of the aerogel composite is between about 25,000 psiand about 46,000 psi.
 10. The aerogel composite of claim 1, whereinaerogel composite comprises a polyimide aerogel or derivative thereof.